Digital Acoustofluidics Based Contactless and Programmable Liquid Handling

Handling of fluids is essential for a majority of applications involving liquid phase reactions in chemistry, biology, and biomedicine. In contrast to manual pipetting in conventional small workshops, automated liquid handling techniques have brought unrivaled accuracy, precision, speed, and repeatability to modern biomedical researches and pharmaceutical industries. Despite their benefits, most advanced liquid handling techniques (e.g., microfluidics and micro-plates) lack fluidic rewritability due to surface-adsorption-induced contaminations on solid-liquid interfaces, limiting their capability of performing complex cascade reactions or high-content combinatorial screening on reusable fluid carriers. To date, the lack of fluidic rewritability still remains as a challenge for engineering scientists to achieve the automated processing of ‘fluidic bits’ in a manner similar to ‘electronic bits’ within a miniature chip. In this work, we approach the fluidic rewritability by contactlessly manipulating aqueous droplets floating on a dense, immiscible carrier fluid layer using acoustic-streaming-induced hydrodynamic gradients. The presented acoustic streaming-based liquid handling (i.e., digital acoustofluidics) devices can be categorized into three versions. (1) The first version of digital acoustofluidic devices actuate floating droplets and small objects by actively propelling them along a straight path following the horizontal direction of acoustic wave propagation. (2) In contrast, the second version employs acousto-hydrodynamic potential traps on the surface of the carrier fluid layer to attract and capture the floating droplets at the equilibrium position of the triggered butterfly-shaped streaming pattern. By selectively exciting the immersed interdigital transducers and sequentially triggering the localized acousto-hydrodynamic traps, the floating droplets can be transported, merged, mixed, split, and generated in a contact-free and programmable manner. (3) The third version of digital acoustofluidic devices is built upon the second version by integrating additional channel-shaped acoustic streaming vortices under high-amplitude excitations, enabling dual-mode manipulation using a single unit transducer. Furthermore, based on the scalable feature of the channel-shaped acoustic streaming vortices, fundamental droplet logic control can be achieved without solid-liquid interactions.

Altogether, this article summarizes the trials-and-errors, working mechanism, design principle, controlling strategy, and potential improvement directions of our digital acoustofluidics platform to facilitate the future development of compact liquid handling workstation with fluidic rewritability. Furthermore, it is our hope that our results and efforts can benefit the explorations in acoustic streaming and associated meso-/micro-manipulation techniques. Lastly, we hope the concept of fluidic rewritability in digitized liquid handling may motivate future microfluidic engineers to develop real Lab-on-a-Chip devices to enable high-speed automation of reactions with dynamic reconfigurability and controllability.